FIG. 4 is an illustration for explaining a power generation mechanism and charging (power storage) mechanism of a regenerative fuel cell system using hydrogen and oxygen. FIG. 4(a) is referred to as a separate type in which a fuel cell section and a water electrolysis section are provided in a separated manner, while FIG. 4(b) is referred to as a unitized type (or “a reversible type”) in which a fuel cell section and a water electrolysis section are provided in an integrated manner.
In the fuel cell system in FIG. 4(a), water is electrolyzed into a hydrogen gas and an oxygen gas in the water electrolysis section during a charging (power storage) time. Each of the resulting hydrogen gas and oxygen gas are stored in a respective tank. During a power generation time, the hydrogen gas and oxygen gas stored in the respective tank are reacted in the fuel cell section to generate electric power. Water obtained at that time is stored in a dedicated tank. In the fuel cell system in FIG. 4(b), the reactions during the charging (power storage) time and the power generation time are performed in a similar fashion, but are different from those of FIG. 4(a) in that the both reactions are performed in a unitized fuel cell section.
During the time of electrolysis of water, an electrode is required to contain moisture, while during the power generation time, it is required to be dried. Therefore, a generic regenerative fuel cell system often employs the separate type as illustrated in FIG. 4(a). On the other hand, in the case where a severe limitation is imposed on weight, such as in a space application, the unitized type has an advantage from the view point of effectiveness against mass.
Further, during a power generation time, a proton (H+) (or hydrogen ion) that is separated from an electron is required to be transported through a polymer membrane to the other side thereof, for which the polymer membrane is required to be moisturized by containing moisture. To satisfy the above demand, the Applicant has proposed to allow the polymer membrane to be appropriately moisturized by flowing the oxygen gas and the hydrogen gas from each of the opposite sides of the polymer membrane to the other side thereof with respect to each other (Patent Document 1). Examples of the fuel cell system in which a fuel cell section and a water electrolysis section are provided in an integrated manner can also be found in Patent Documents 2 and 3.